The present invention relates to an improved method and apparatus for preparing low-concentration polysilicate microgels, i.e., aqueous solutions having an active silica concentration of generally less than about 1.0 wt. %, which are formed by the partial gelation of an alkali metal silicate or a polysilicate, such as sodium polysilicate, having in its most common form one part Na.sub.2 O to 3.3 parts SiO.sub.2 by weight. The microgels, which are referred to as "active" silica in contrast to commercial colloidal silica, comprise solutions of from 1 to 2 nm diameter linked silica particles which have a surface area of at least about 1000 m.sup.2 /g. The particles are linked together during preparation, i.e., during partial gelation, to form aggregates which are gaged into three-dimensional networks and chains. The polysilicate microgels can be further modified by the incorporation of aluminum oxide into their structure. Such alumina modified polysilicates are classified as polyaluminosilicate microgels and are readily produced by a modification of the basic method for polysilicate microgels. A critical aspect of the invention is the ability to produce the microgels within a reasonable time period, i.e., not longer than about 15 minutes until the microgel is ready for use, without the risk of solidification and with minimum formation of undesirable silica deposits within the processing equipment. In this connection, the incorporation of alumina into the polysilicate microgel has been found beneficial in that it increases the rate of microgel formation. Polysilicate microgels produced according to the invention are particularly useful in combinations with water soluble cationic polymers as a drainage and retention aid in papermaking. At low pH values, below pH of 5, these products are more appropriately referred to as polysilicic acid microgels. As the pH value is raised, these products can contain mixtures of polysilicic acid and polysilicate microgels; the ratio being pH-dependent. For sake of convenience, these products hereinafter will be referred to as polysilicate microgels.